The present invention relates to a light-emitting device like a semiconductor laser device, and more particularly relates to a semiconductor light-emitting device for emitting radiation in the ultraviolet to blue regions. The present invention also relates to a method for fabricating the semiconductor light-emitting device and to an optical disk apparatus using the light-emitting device.
In recent years, semiconductor light-emitting devices that can emit radiation at short wavelengths ranging from the ultraviolet to blue regions, or semiconductor laser devices, in particular, have been researched and developed vigorously. This is because such light-emitting devices are expected to further increase the recording density of optical disks or the resolution of laser printers and are applicable to optical measuring instruments, medical equipment, display devices, illuminators and so on.
Examples of semiconductor materials that can emit radiation at such short wavelengths include Group III nitride semiconductors. For instance, a semiconductor laser device with a multiple quantum well active layer, which is a stack of silicon (Si)-doped GaInN/GaInN layers, can oscillate continuously at a wavelength of about 401 nm and at room temperature and can operate for as long as about 3,000 hours under the conditions that the ambient temperature is 20xc2x0 C. and the output power thereof is 2 mW. See Japanese Journal of Applied Physics, Vol. 36 (1997), pp. 1568-1571, for example.
Group III nitride semiconductor crystals are generally grown by a metalorganic vapor phase epitaxy (MOVPE) process. For example, Japanese Laid-Open Publication No. 6-196757 discloses a method of growing a semiconductor layer of GaInN of excellent crystal quality on a semiconductor layer of GaN by using nitrogen as a carrier gas.
The known method of producing a Group III nitride semiconductor, however, is disadvantageous in that pits are created in the GaInN/GaN multiple quantum well structure thereof (to be an active layer) at as high a density as 108 to 109 cmxe2x88x922 as described in Applied Physics Letters, Vol. 72 (1998), pp. 710-712, for example.
Those pits adversely affect the operation characteristics of a light-emitting device, e.g., raises the threshold value, at which the laser device starts to oscillate, or lowers the reliability thereof. This is because the existence of the pits not only decreases the luminous efficacy, but also causes localized levels by making the composition of In non-uniform, constitutes a source of diffusion of In being grown or results in scattering or absorption loss in an optical waveguide.
To obtain a Group III nitride semiconductor light-emitting device, or semiconductor laser device, in particular, with characteristics practically applicable to an optical disk apparatus, for example, the composition of In within the GaInN well layer thereof should be uniformized. In addition, each multiple quantum well layer should be of uniform quality and be sufficiently planarized.
Moreover, the structure of the device should be modified such that electrons, which are injected from an n-type conductive layer into the quantum well layer, can be injected into the active layer efficiently and uniformly without overflowing into a p-type conductive layer during the operation of the device.
An object of the present invention is solving the problems of the prior art to suppress the creation of pits in quantum well layer containing indium and nitrogen in a Group III nitride semiconductor light-emitting device and to inject electrons into the quantum well layer more efficiently.
To achieve this object, the present invention takes the following measures:
1) Each barrier layer included in the quantum well layer contains aluminum;
2) The stress vector of each barrier layer is of the opposite sign to that of each well layer;
3) Only one of barrier layers that is in contact with a p-type conductive layer contains aluminum in multiple quantum well layer;
4) In growing the quantum well layer by an MOVPE process, triethylgallium is used as a gallium source.
The present inventors analyzed how inverted hexagonal parallelepiped pits with {1-101} planes as facets are formed at a high density in a GaInN/GaN or GaInN/GaInN multiple quantum well structure in accordance with a conventional fabrication process. As a result, we reached the following conclusions.
To relax a compressive strain induced in a GaInN layer or a strain resulting from localized segregation of In, nuclei of pits are created at more than a critical thickness. In addition, at the growth temperature of the GaInN layer (usually at a growth temperature of about 800xc2x0 C.), the growth rate for {1-101} planes is lower than that for the (0001) plane in the GaInN layer. Accordingly, as the crystals are growing, the pits are also increasing their sizes. Those pits, which have been created in the GaInN layer, are gradually filled in and the surfaces of the crystals are planarized while an optical guide layer, a cladding layer and so on are grown one upon the other on the GaInN layer at a growth temperature of about 1000xc2x0 C. This is because the growth rate for the {1-101} planes is higher than that for the (0001) plane in the optical guide layer, etc.
It should be noted that when a zone axis index or a Miller index representing a crystallographic plane orientation is followed by a negative sign, the index following the negative sign is a negative direction index in this specification.
The present inventors examined various methods of suppressing the creation of those pits. As a result, we made the following findings.
Specifically, if the multiple quantum well structure includes an aluminum (Al)-containing barrier layer, then a tensile strain is induced in the barrier layer, and a compressive strain applied to the multiple quantum well structure decreases. Consequently, the critical thickness increases.
In addition, the existence of Al with high electric field intensity in a crystal reduces the diffusion of In, thus suppressing the segregation of In, which strongly tends to segregate locally.
Moreover, the growth rate of the Al-containing semiconductor layer, i.e., an AlGaN layer, for the {1-101} planes is not so different from that for the (0001) plane compared to the GaInN layer. Accordingly, the expansion of pits can be reduced.
Furthermore, if the In mole fraction in the well layer is 0.1 or less, the total thickness of the multiple quantum well structure does not exceed the critical thickness.
Furthermore, if the strain vector of the barrier layer is of the sign opposite to that of well layer, then the total strain applied to the multiple quantum well structure can be reduced, thus increasing the critical thickness.
Also, if triethylgallium (TEG) is used a gallium source in forming the multiple quantum well structure, then the growth rate for the (0001) plane is not so different from that for the {1-101} planes in the quantum well structure.
Accordingly, the expansion of pits can be reduced.
As for a method for injecting electrons more efficiently, we made the following findings.
If the multiple quantum well structure includes a barrier layer with a strain vector of the opposite sign to that of each well layer, then the total strain induced in the multiple quantum well structure can be smaller. Thus, the intensity of a piezoelectric field induced in the multiple quantum well structure decreases. As a result, electrons are injected into the well layers more uniformly.
Alternatively, if only one barrier layer in contact with a p-type conductive layer contains Al and the other barrier layers, which are not in contact with the p-type conductive layer, do not contain Al, then the electrons injected into the well layers do not overflow into the p-type conductive layer. As a result, the electrons can be injected into the well layers more efficiently.
Specifically, a first semiconductor light-emitting device according to the present invention is made of Group III-V compound semiconductors. The device includes a quantum well layer, which is formed over a substrate and includes a barrier layer and a well layer that are alternately stacked one upon the other. The band gap of the well layer is narrower than that of the barrier layer. And the well layer contains In and N, while the barrier layer contains Al and N.
In the first semiconductor light-emitting device, the barrier layer contains Al and N. That is to say, if Al is contained in the barrier layer, a tensile strain is induced in the barrier layer to fill in the pits, which are created at more than the critical thickness to relax the compressive strain induced in the well layer. As a result, the compressive strain induced in the quantum well layer decreases and the critical thickness increases. Also, since Al is contained in the barrier layer, the In segregation in the well layer can be suppressed. Moreover, the growth rate for the {1-101} planes is not so different from that for the (0001) plane compared to a well layer containing In. Accordingly, the expansion of pits can be suppressed, thus reducing the threshold current of the light-emitting device and greatly improving the reliability of the device.
In the first semiconductor light-emitting device, a plurality of the barrier layers are preferably provided between p- and n-type conductive layers. One of the barrier layers that is in contact with the p-type conductive layer preferably has an aluminum mole fraction larger than that of the other barrier layer(s) that is/are not in contact with the p-type conductive layer. In such an embodiment, since Al is added to the barrier layer in contact with the p-type conductive layer, the barrier layer has its heterobarrier increased. Thus, it is possible to prevent electrons, which have been externally injected, from overflowing into the p-type conductive layer without being injected into the well layers. As a result, the electrons can be injected into the well layers more efficiently.
In this particular embodiment, the aluminum mole fraction of the one barrier layer in contact with the p-type conductive layer preferably increases from a part thereof closest to the n-type conductive layer toward another part thereof closest to the p-type conductive layer. In such an embodiment, the hole density in the barrier layer in contact with the p-type conductive layer can be decreased, thus increasing the efficiency with which holes are injected into the well layers.
In the first semiconductor light-emitting device, the well layer is preferably made of gallium indium nitride (GaInN) or aluminum gallium indium nitride (AlGaInN), while the barrier layer is preferably made of aluminum gallium nitride (AlGaN). In such an embodiment, the creation of the pits can be suppressed with much more certainty in the quantum well layer.
A second semiconductor light-emitting device according to the present invention is made of Group III-V compound semiconductors. The device includes a quantum well layer, which is formed over a substrate and includes a barrier layer and a well layer that are alternately stacked one upon the other. The band gap of the well layer is narrower than that of the barrier layer. The barrier layer has a strain vector of a sign opposite to that of a strain vector of the well layer.
In the second semiconductor light-emitting device, since the strain vectors of the barrier and well layers are of mutually opposite signs, the strain quantities in the quantum well structure are canceled by each other and decrease. Accordingly, the critical thickness, at which pits are created, increases and in addition, the piezoelectric field induced in the quantum well structure decreases. As a result, electrons and holes are injected into each well layer uniformly, thus increasing the luminous efficacy.
In the second semiconductor light-emitting device, the well layer preferably contains In and the barrier layer preferably contains Al.
The first or second semiconductor light-emitting device preferably further includes first and second optical guide layers. The first optical guide layer is provided on one side of the quantum well layer that is closer to the substrate, while the second optical guide layer is provided on another side of the quantum well layer that is opposite to the substrate. The band gap of the barrier layer is preferably smaller than or equal to that of the first and second optical guide layers.
Also, an In mole fraction of the well layer is preferably larger than 0 and equal to or smaller than 0.1. In such an embodiment, it is possible to prevent the total thickness of the quantum well layer from exceeding the critical thickness. As a result, the creation of the pits can be suppressed with much more certainty in the quantum well layer.
Moreover, the barrier layer or the well layer preferably contains silicon (Si) as a dopant. In such an embodiment, it is possible to prevent In from locally segregating in the quantum well layer, thus relaxing the strain resulting from such local segregation of In. As a result, the creation of the pits, which are formed to reduce the strain, can be suppressed.
A third semiconductor light-emitting device according to the present invention is made of Group III-V compound nitride semiconductors. The device includes: a quantum well layer, which is formed over a substrate and includes a plurality of barrier layers and a well layer that are alternately stacked one upon the other, the band gap of the well layer being narrower than that of each said barrier layer; and p- and n-type conductive layers formed over the substrate to vertically interpose the quantum well layer therebetween. One of the barrier layers that is in contact with the p-type conductive layer contains aluminum, while the other barrier layer(s) that is/are not in contact with the p-type conductive layer contain(s) no aluminum.
In the third semiconductor light-emitting device, a large heterobarrier, which is caused by the addition of Al, exists between the barrier layer in contact with the p-type conductive layer and the well layer in contact with the barrier layer on the opposite side to the p-type conductive layer. Thus, it is possible to prevent electrons from going over the well layer to overflow into the p-type conductive layer. In addition, since a piezoelectric field is induced in the direction in which the overflow of the electrons over the well layer is suppressed, the electrons can be injected into the well layer more efficiently.
In the third semiconductor light-emitting device, the well layer preferably contains In.
In the third semiconductor light-emitting device, the well layer is preferably made of GaInN, the barrier layer in contact with the p-type conductive layer is preferably made of AlGaN, and the other barrier layer(s) is/are preferably made of GaInN or GaN.
An inventive method for fabricating a semiconductor light-emitting device is adapted to fabricate a semiconductor light-emitting device of Group III-V compound semiconductors, in which a quantum well layer including barrier and well layers is formed by a metalorganic vapor phase epitaxy process over a substrate by alternately stacking the barrier and well layers one upon the other. The band gap of the well layer is narrower than that of the barrier layer. The method includes the steps of: forming the barrier layer, which contains gallium (Ga) and nitrogen (N), over the substrate by using at least gallium and nitrogen sources as first source materials; and forming the well layer, which contains Ga, In and N, on the barrier layer by using at least gallium, indium and nitrogen sources as second source materials. The gallium source used in the steps of forming the barrier layer and the well layer is triethylgallium (TEG).
According to the inventive method for fabricating a semiconductor light-emitting device, TEG is used as the gallium source in the steps of forming the barrier layer and the well layer. Thus, the growth rate of the quantum well layer for the {1-101} planes is not so different from that for the (0001) plane. Accordingly, the expansion of pits can be suppressed, thus improving the crystal quality of the well layers. As a result, the operating performance of the device can be improved.
In the inventive method for fabricating a semiconductor light-emitting device, the step of forming the barrier layer preferably includes the step of forming the barrier layer of AlGaN by using an aluminum source as an additional one of the first source materials. The step of forming the well layer preferably includes the step of forming the well layer of either GaInN or AlGaInN by using an aluminum source as an additional one of the second source materials. Also, an In mole fraction of the well layer is preferably larger than 0 and equal to or smaller than 0.1.
An optical disk apparatus according to the present invention includes: the semiconductor light-emitting device according to any of the first through aspects of the present invention; a condensing optical system for condensing outgoing radiation, which has been emitted from the semiconductor light-emitting device, on a storage medium on which data has been recorded; and a photodetector for receiving light that has been reflected from the storage medium.
In the inventive optical disk apparatus, the photodetector preferably reads the data that has been recorded on the storage medium based on a reflected part of the outgoing radiation.
In this case, the photodetector is preferably provided near the semiconductor light-emitting device.
In this particular embodiment, the photodetector is preferably provided on a principal surface of a support member made of silicon, and the semiconductor light-emitting device is preferably supported on the principal surface of the support member.
In such a case, the principal surface of the support member is preferably provided with a concave portion with a micro mirror on a sidewall thereof. And the semiconductor light-emitting device is preferably secured to the bottom of the concave portion of the support member such that the outgoing radiation emitted from the semiconductor light-emitting device is reflected from the micro mirror and advances substantially vertically to the principal surface of the support member.